does not seem to be any capacitys in the hand book.

any idea anybody ?

My 2013 Roomster has a 55 litre tank

The July 2009 Roomster catalogue (S, 2, SE & Scout) confirms Eccleshill's post.

Tank capacity is 55 litres (12 gallons in old money).

That's equivalent to 600+ miles per tankful on my 1.9TDI

55l stock, if you press or remove the fuel tank vent valve (google "ventectomy") it becomes 64l. Safe to do on a diesel only.